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This study presents the basic ingredients needed to model the out-of-plane compression behaviour of a folded core up to the global peak load. This core is made of an aramid paper. The proposed approach is both experimental and numerical and handles the problem at different scales: fibre, paper and core scales. To have a good representation of the physics, two key points are essential. The first is the mechanical behaviour of the paper. The aramid paper can be modelled as a laminate composite. An experimental study of the paper behaviour in bending has been conducted to build a damage model for the paper. The second key point concerns the geometrical defects of the folded core. The defects, arising from the manufacturing process, are built by simulation of the folding process. The macroscopic behaviour of the core under out-of-plane compression loading is shown to be very sensitive to these defects. The results of a simulated crushing test are compared to experimental ones. A good agreement is shown. [ABSTRACT FROM AUTHOR]

The paper presents a strain measurement technique to be applied on repetitive knitted nets. The application of the procedure is focused on large deployable reflectors used in space applications. Manufacturing of these structures involves knitting of thin metallic wires. Besides space antennas, the method herein developed is advantageous for many other types of knitted tissues such as elastic coatings, elastic bandages for medical applications, or any other woven fabric requiring a defined elastic behaviour. All of them display an almost regular repetitive pattern whose variations due to imposed loads are the core of the method. Deformation of such kind of structural elements must be analysed in two dimensions, because the knitting causes an accentuated orthotropic behaviour. Strain measurement is tricky and requires special computational techniques. Indeed, the application of standard experimental methods and indirect strain determination, through well-established image analysis algorithm, can be totally unsuccessful or unsuitable because of the lack of continuity of these structures. The paper presents a method based on two-dimensional Fourier analysis. Numerical examples, supported by some experimental evidences, confirm the feasibility of the technique. Furthermore, a newly proposed frequency zooming algorithm, based on fast Fourier transform, makes the computational costs of the indirect strain measurement affordable. [ABSTRACT FROM AUTHOR]

The article discusses various reports published within the issue, including one by C. A. Sciammarella and colleagues on the use of surface plasma to characterise contact stress distributions and surface profiles, one by M. Beghini and colleagues on the application of the influence functions to address inverse problems, and the one by N. Bonora on the measurement of the damage with the use of material Young's modulus degradation.

In responding to the needs of the material characterization community, the recently developed mesh-free random grid method (MFRGM) has been exhibiting very promising characteristics of accuracy, adaptability, implementation flexibility and efficiency. To address the design specification of the method according to an intended application, we are presenting a sensitivity analysis that aids into determining the effects of the experimental and computational parameters characterizing the MFRGM in terms of its performance. The performance characteristics of the MFRGM are mainly its accuracy, sensitivity, smoothing properties and efficiency. In this paper, we are presenting a classification of a set of parameters associated with the characteristics of the experimental set-up and the random grid applied on the specimen under measurement. The applied sensitivity analysis is based on synthetic images produced from analytic solutions of specific isotropic and orthotropic elasticity boundary value problems. This analysis establishes the trends in the performance characteristics of the MFRGM that will enable the selection of the user controlled variables for a desired performance specification. [ABSTRACT FROM AUTHOR]

This paper is concerned with the study of temperature influence on Young's modulus, ultimate strength and fracture toughness properties of PVC/CaCO particulate composites with different volume fractions. The tests were performed in three- and four-point bending. The resonant technique was also used to analyse the influence of both volume fraction and temperature on Young's modulus. Significant decrease of ultimate strength, fracture toughness and Young's modulus was observed with the increase of the temperature. Ultimate strength decreases with the increase of particle volume fraction at room temperature. For the other temperatures, this decreasing trend is less clear. PVC/CaCO flexural Young's modulus calculated for a much lower loading segment increases with volume fraction. The same trend was obtained using the resonant technique. However, as the loading segment used to calculate the Young's modulus was increased a significant decrease of Young's modulus was obtained as a result of a progressive debonding at the particle-matrix interface. A 2D simplified FE simulation also confirms such trend. The dependence of Young's modulus relatively to the loading segment increases as the volume fraction is increased, leading to composite Young's modulus below matrix value for higher volume fractions and higher loading segments. Fracture toughness decreases with volume fraction. [ABSTRACT FROM AUTHOR]

The response of three different cellular core types, suitable for manufacturing crashworthiness sandwich cellular structures, is investigated in this paper. A methodology is developed, comprising linear static and eigenvalue buckling analysis, as well as nonlinear material elastoplastic analysis. The methodology is used to study the structural response and failure process of open lattice metallic cellular cores and derive the most important structural properties of the cellular core, i.e. elasticity modulus, plateau stress and compaction strain. The critical elastoplastic buckling stress of the metallic struts is approximated by analytical solutions, while a simple engineering approach is applied for the estimation of the compaction strain. The influence of core basic design parameters, i.e. strut aspect ratio (radius/length), unit-cell spatial configuration and unit-cell size on the structural behaviour is assessed. [ABSTRACT FROM AUTHOR]

This paper focuses on the investigation of the three-point bending tensile strength of Dionysos marble. Using the classical relationship obtained from the theory of elasticity, the effect of the span length, the thickness/width ratio and the orientation of the specimens in the indirect tensile strength were investigated. In total, 42 experiments were carried out, divided in three sets of specially designed beams. Finally, the calculated bending tensile strength is compared with the corresponding value obtained from Brazilian indirect tension as well as ring tests. [ABSTRACT FROM AUTHOR]

This paper presents the use of a sensitivity study to find the best combinations of free length and fibre angle in an unnotched Iosipescu test processed with the virtual fields method. The sensitivity to noise coefficients arising from the special virtual fields procedure are used to build up a cost function. This function is aimed at balancing out the coefficients of the different orthotropic stiffnesses so that the same confidence level can be reached on all these parameters. Then, experimental validation was performed using a speckled interferometry (ESPI) system. Full-field strains were measured and stiffnesses identified and compared between the usual 0°, 30 mm configuration and an improved 25°, 40 mm configuration. The outcome of the optimisation was confirmed by testing the same specimen several times and comparing scatter between the two configurations. This is a first promising result on the way to the design of a new test for orthotropic stiffness identification on a single specimen from full-field measurements. [ABSTRACT FROM AUTHOR]

In this paper, a new fully automated photoelastic technique based on both the Fourier transform and the envelope of the fringe centres, is presented. Using two images of the isochromatics acquired with different loads and a simple calculation routine, the method allows the user the complete determination of the isochromatic fringe retardation on any arbitrary line chosen over the model domain, without the need for supplementary external information or calibration. Various experiments have corroborated the accuracy of the method, estimated theoretically to about 0.07 fringe orders. As a result of its simplicity and sufficient accuracy, the method can be used in the industrial field also by a non-expert user. [ABSTRACT FROM AUTHOR]

A semi-analytical approach is taken to calculate the geometric contribution to gauge factor for patterned lines on substrates. A finite element model is presented in which a line section is strained along its length, and cross-sectional dilatation is calculated for different line aspect ratios, and elastic properties of line and substrate. Results are bounded by analytical solutions for infinitely wide and tall lines. A simple experiment is posed for measuring Poisson's ratio and piezoresistivity of coatings, films or patterned structures. [ABSTRACT FROM AUTHOR]

Current high strain rate testing techniques typically rely on the split‐Hopkinson bar (SHB). The early response in an SHB test is corrupted by inertia making it difficult to accurately characterise the transition from elasticity to plasticity for metals. Therefore, a new test method is required. This article is the second in a two‐part series which aims at developing a new high strain rate test for elasto‐plasticity identification using the image‐based inertial impact (IBII) method. The goal of this article is to validate the new method experimentally using IBII tests on aluminium 6082‐T6 (minimal rate sensitivity) and stainless steel 316L (rate sensitive). Comparison of the quasi‐static and dynamic stress–strain curves for the aluminium case showed minimal difference providing experimental validation of the method. The same comparison for the steel showed that the method was able to detect rate sensitivity. [ABSTRACT FROM AUTHOR]

In vivo measurement of the mechanical properties of soft tissues is essential to provide necessary data in biomechanics and medicine (early cancer diagnosis, study of traumatic brain injuries, etc.). Imaging techniques such as magnetic resonance elastography can provide 3D displacement maps in the bulk and in vivo, from which, using inverse methods, it is then possible to identify some mechanical parameters of the tissues (stiffness, damping, etc.). The main difficulties in these inverse identification procedures consist in dealing with the pressure waves contained in the data and with the experimental noise perturbing the spatial derivatives required during the processing. The optimised virtual fields method (OVFM) (), designed to be robust to noise, presents natural and rigorous solution to deal with these problems. The OVFM has been adapted to identify material parameter maps from magnetic resonance elastography data consisting of 3D displacement fields in harmonically loaded soft materials. In this work, the method has been developed to identify elastic and viscoelastic models. The OVFM sensitivity to spatial resolution and to noise has been studied by analysing 3D analytically simulated displacement data. This study evaluates and describes the OVFM identification performances: Different biases on the identified parameters are induced by the spatial resolution and experimental noise. The well-known identification problems in the case of quasi-incompressible materials also find a natural solution in the OVFM. Moreover, an a posteriori criterion to estimate the local identification quality is proposed. The identification results obtained on actual experiments are briefly presented. [ABSTRACT FROM AUTHOR]

ABSTRACT Due to its excellent bioactivity, 45S5 Bioglass® is being highly considered in tissue engineering scaffold development. In order to enhance vascularization promoting tissue growth, these scaffolds typically have a highly interconnected porous structure with a porosity between 80 and >90%. Often, Bioglass®-based scaffolds of such a high porosity have insufficient stiffness. In order to increase the stiffness of Bioglass®-based scaffolds fabricated by the foam replica method, the herein investigated scaffolds were coated with a number of different biopolymers, including: collagen, gelatin, polycaprolactone (PCL), alginate and poly( l-lactic acid). The resulting stiffness gain was quantified by means of ultrasonic measurements. Accordingly, PCL and collagen coatings increased the scaffold stiffness, as compared to uncoated scaffolds, by 58 and 38%, respectively; while no remarkable stiffness increase was recorded for the other coatings. Additionally, scanning electron microscopy images of polymer coated scaffolds revealed that PCL coatings had not clogged the scaffold's micropores, which is deemed essential for cell seeding and to enable in-growth of bone tissue. Thus, the application of PCL coatings represents a promising strategy for mechanical competence enhancement of Bioglass®-based scaffolds for bone tissue engineering. [ABSTRACT FROM AUTHOR]

ABSTRACT A non-linear optimization procedure is established to determine the elastic modulus of slender, soft materials using beams with unknown initial curvature in the presence of large rotations. Specifically, the deflection of clamped-free beams under self-weight - measured at different orientations with respect to gravity - is used to determine the modulus of elasticity and the intrinsic curvature in the unloaded state. The approach is validated with experiments on a number of different materials - steel, polyetherimide, rubber and pig skin. Because the loading is limited to self-weight, the strain levels attained in these tests are small enough to assume a linear elastic material behaviour. This non-destructive methodology is also applicable to engineered tissues and extremely delicate materials in order to obtain a quick estimate of the material's elastic modulus. [ABSTRACT FROM AUTHOR]

ABSTRACT Rate-dependent models require creep or mechanical tests at various strain rates in order to be identified and validated. Different geometries coexist for creep and static tests (normative geometry) and for dynamic tests. Therefore, due to geometrical sample considerations, experimental results could be inconsistent for identification or validation procedures, inducing, for example, differences on the shear modulus only due to the change of geometry. The objective of this work is to present an improved sample geometry that allows to obtain consistent mechanical tests results at various strain rates highlighting the rate dependencies of laminates. In particular, a complete mechanical validation of the sample geometry for dynamic tests is successfully performed in order to avoid inconsistency. Results of static and dynamic tests on the validated geometry are analysed, and the rate dependency of the elastic properties of the UD T700GC/M21 mesoscopic ply is highlighted on a wide strain rate range (10−3 to 102 s−1). Finally, the identification of a non-linear viscoelastic model is performed on dynamic and creep tests results in order to obtain a representative model for dynamic, static and creep loadings, and to demonstrate the importance of introducing the improved geometry for the dynamic tests. [ABSTRACT FROM AUTHOR]

The shear response of irregular open-lattice cellular cores made of interconnected metallic struts is analysed, and the core shear modulus in the three material principal directions is derived. The analytical approach is based on the technical beam theory, which is used for the determination of the unit-cell response under complex loading. The influence of the strut geometrical parameters and unit-cell shape and size on the cellular core shear stiffness is determined. The analytical determination of the unit-cell properties is successfully validated by a reference numerical model of the unit cell, which is developed for comparison purposes. Furthermore, the homogenisation principles are applied to the prediction of the shear response of a core block structure comprising a high number of unit cells, for which experimental results were available; the comparison revealed that the experimental results coincide well with the results obtained from the homogenised model. [ABSTRACT FROM AUTHOR]

A detailed numerical and experimental study of a NiTi Shape Memory Alloy (SMA) wire subjected to bending is presented in order to provide a complete characterisation of the material under this load case. The beam model presented is based on the classic Euler-Bernoulli theory and uses De la Flor one-dimensional constitutive equations modified to take into account different material responses to tension and compression as well as the different elastic properties of austenite and martensite. All the necessary experimental techniques were designed to determine the bending behaviour of NiTi SMA wire over the entire range of transformation temperatures. These experimental bending results were then compared with the numerical results. The numerical model proposed agrees quantitatively and qualitatively with the experimental bending results obtained for NiTi wire, representing an effective tool for the analysis of one-dimensional structural devices. A comparison of the numerical results that assume symmetrical behaviour between tension and compression with the results that assume asymmetrical behaviour has shown that the tension/compression asymmetry is more pronounced in the martensitic range and has little influence on the response of the SMA wire subjected to bending at temperatures above A. These results are analyzed and discussed. [ABSTRACT FROM AUTHOR]

In short glass fibre reinforced polymers parts obtained by injection moulding, processing conditions produce complex orientation patterns that influence the mechanical properties of the component. In this work, the problem of whether a morphological description can be predictive of the elastic behaviour of these materials is addressed, with particular reference to the shell and core zones identified in a PA6-GF 30 sample. Morphological parameters considered include fibre volume fraction, Mean Intercept Length (MIL) fabric tensor eigenvalues and principal directions. Apparent elastic moduli values in different sites within the sample were computed by simulations using a numerical model based on the Cell Method. Both morphological and numerical methods were able to capture the principal directions of anisotropy and the differences between shell and core and among the volumes of interest within each region. Even if the fabric tensor, normalised, eigenvalues did not show a significant correlation to the apparent elastic moduli, approximately 92% of stiffness components variance could be accounted for by changes in MIL eigenvalues once normalised values were used for elastic moduli as well. [ABSTRACT FROM AUTHOR]

Experimental results obtained in a series of displacement controlled oedometric tests on soft rocks are presented. Four different materials characterised by a high void ratio have been examined; three natural soft rocks and an artificial one. The materials under investigation were conchyliates stone, calcarenite, pumice stone and Gasbeton. In order to monitor the stress path as well, a soft oedometer ring was designed and constructed for the measurement of the radial stresses. The observed behaviour can be divided in three phases. After an initial phase in which the mechanical response is essentially elastic, a second phase starts, in which bonds are progressively broken, so that in the axial stress–strain curve a stress decrease is recorded and in some cases, a sort of ‘curl’ appears in the stress path. This is associated to the occurrence of strain non-homogeneities in the form of compaction bands. In the final part of the experiment the axial stress increases exponentially and the stress path returns to be linear, as the one expected for a cohesionless material. The experimental behaviour is reproduced by means of an elastoplastic strain-hardening/softening constitutive model and the occurrence of compaction bands is theoretically predicted. [ABSTRACT FROM AUTHOR]

Ni47Ti53 films of varying thickness were deposited onto Ni56Ti44 substrates. Annealing the films produced compositional gradients through diffusion modification. Nanoindentation measurements were used to probe the mechanical properties at various depths into the film. The films exhibited a variation in elastic modulus as a function of indentation depth that depended on the thickness of the film. A self-similarity principle permitted the mechanical properties of the graded NiTi films to be resolved further into substrate, beyond the contact depth of the tip. The observed variation of elastic modulus with indentation depth in the graded NiTi films was considered to be a combined response from changes in microstructure, substrate effects, and mechanically-induced phase transformations. This variation was predicted using an analysis that accounted for the transformation effects occurring under the tip during loading and the graded distribution of martensite volume fraction obtained from diffusion modification. [ABSTRACT FROM AUTHOR]

Mechanical testing is the most common experimental technique to determine elastic stiffness of materials. In case of porous materials, especially such with very high porosity, the determination of material stiffness may be strongly biased by inelastic deformations occurring in the material samples, especially in the vicinity of the load transfer devices, such as loading platens. In contrast, ultrasonic waves propagating through a material generate very small stresses and strains (and also strain rates lying in the ‘quasistatic’ regime). Thus, they enable the direct determination of the components of elastic stiffness tensors of materials, and also of those with a very high porosity. We shortly revisit from the theoretical basis of continuum (micro)mechanics that, depending on the frequency of the employed acoustical signals, the investigated materials are characterised at different observation scales, e.g. the elasticity of the overall porous medium, or that of the solid matrix inside the material are determined. We here report the elastic properties of biomaterials and biological materials at different length scales, by using ultrasound frequencies ranging from 100 kHz to 20 MHz. We tested isotropic scaffolds for biomedical engineering, made up of porous titanium and two different bioactive glass–ceramics, and we also determined the direction-dependent normal and shear stiffness components of the anisotropic natural composite ‘spruce wood’. [ABSTRACT FROM AUTHOR]

Investigation into the quasi-static simple shear of shape memory alloy (SMA) was carried out. A special grip was designed which allowed replacing compression into the shear process on testing machine. At the same time the shear zone temperature was observed by an infrared camera. The mechanical and thermal characteristics of sheet specimens of NiTi subjected to superelastic shear deformation due to reversible stress-induced phase transformation were clarified. By comparison of the data, the processes of martensite and reverse transformations during the shear deformation were analysed. The investigation shows that stages of the phase transformations during the simple shear process are similar like those observed during tension test. Moreover, during the simple shear deformation no uniform temperature distributions were noticed, especially at higher shear rate, manifesting that the stress-induced phase transformation during the shear process is also inhomogeneous. The thermomechanical properties of the NiTi SMA for various shear rates were investigated. It was found that an increase in the strain rate results in an increase in temperature variation, the shear stresses and the magnitude of hysteresis loops between the forward and reverse transformations. [ABSTRACT FROM AUTHOR]

The increasing use of low-modulus materials, on which the reinforcement effect of the electrical resistance strain gauge is not negligible, has re-opened the research interest into this issue. This study deals with the evaluation of stiffness, and of the strain gauge Young's modulus involved in the estimation of both the global and the local reinforcement effect; the relationship between the strain gauge stiffness and the local reinforcement effect is also analysed. In particular, the experimental technique used to determine the stiffness of some commercial strain gauges is described. The results show that the strain gauge stiffness alone does not permit an accurate evaluation of the local reinforcement effect. [ABSTRACT FROM AUTHOR]

A fatigue crack in steel (CCT geometry) is studied via digital image correlation. The measurement of the stress intensity factor (SIF) change during one cycle is performed using a decomposition of the displacement field onto a tailored set of elastic fields. The same analysis is performed using two different routes, namely, the first one consists in computing the displacement field using a general correlation technique providing the displacement field projected onto finite element shape functions, and then analysing this displacement field in terms of the selected mechanically relevant fields. The second strategy, called integrated approach, directly estimates the amplitude of these elastic fields from the correlation of successive images. Both procedures give consistent results, and offer very good performances in the evaluation of the crack tip position (uncertainty of about 20 μm for a 14.5-mm crack), SIFs (uncertainty <1 MPa ) and opening properties. [ABSTRACT FROM AUTHOR]

An experimental investigation was conducted to determine the crack growth characteristics and critical tearing energy of pure tyre rubber under mode-I and -III loading. Constrained tension and trousers specimens were used, respectively. In the trousers test it was observed that the crack does not propagate at a steady rate, but in a stick–slip way, i.e., it is arrested and re-initiated at fairly regular intervals. Thus, the force necessary to propagate the crack varies widely from a maximum value at crack initiation to a minimum value at crack arrest. In the constrained tension tests, no stable crack growth was observed and crack initiation coincides with catastrophic fracture. The critical tearing energies corresponding to crack initiation and arrest under mode-III and to unstable crack growth under mode-I loading were determined from the load–displacement records by an approximate analysis of the trousers and the constrained tension tests. [ABSTRACT FROM AUTHOR]

The crystallinity of stretched crystallizable rubbers is classically investigated using X-ray diffraction. In this study, we propose a new method based on temperature measurement and quantitative calorimetry to determine rubber crystallinity during mechanical tests. For that purpose, heat power density are first determined from temperature variation measurements and the heat diffusion equation. The increase in temperature due to strain-induced crystallization is then deduced from the heat power density by subtracting the part due to elastic couplings. The heat capacity, the density, and the enthalpy of fusion are finally used to calculate the crystallinity from the temperature variations due to strain-induced crystallization. The characterization of the stress-strain relationship and the non-entropic contributions to rubber elasticity is not required. This alternative crystallinity measurement method is therefore a user-friendly measurement technique, which is well adapted in most of the mechanical tests carried out with conventional testing machines. It opens numerous perspectives in terms of high speed and full crystallinity field measurements. [ABSTRACT FROM AUTHOR]